Mental Addition of Two-Digit Numbers

This is a complete lesson with instruction and exercises for the student about adding two 2-digit numbers mentally, meant for 2nd grade. The basic idea is to add in parts, such as tens and ones separately, or add in parts in some other way.

Example 1. Add in parts40 + 55.

First break 55 into its
tens and ones. 55 is 50 + 5.

So, 40 + 55 becomes 40 +
50 + 5.

Now add 40 and 50. You get 90. Then add the
5. You get 90 + 5 = ______.

Example 2. Add in parts36 + 30.

First break 36 into three
tens and ones. 36 is 30 + 6.

So, 36 + 30 becomes 30 +
6 + 30.

Now add 30 and 30. That is 60. Then add the
6. You get 60 + 6 = _______.

1. Add in
parts, breaking the second number
into its tens and ones.

a. 20 + 34 =
_______

20 + 30 + 4

b. 70 + 18 = _______

70 + ______ + ____

c. 50 + 27 = _______

50 + ______ + ____

2. Add
in parts. Break the number that is not whole tens into its tens and ones
in your mind.

a.

17 + 10 =
_______

26 + 10 = _______

42 + 10 = _______

b.

16 + 20 =
_______

34 + 30 = _______

67 + 20 = _______

c.

50 + 14 =
_______

60 + 23 = _______

30 + 45 = _______

3. Add mentally. We already studied these. The first one is the helping problem.

a.

7 + 8 = ______

17 + 8 = ______

37 + 8 = ______

b.

4 + 9 = ______

14 + 9 = ______

44 + 9 = ______

c.

8 + 4 = ______

48 + 4 = ______

78 + 4 = ______

d.

7 + 9 = ______

57 + 9 = ______

37 + 9 = ______

How can you easily add 16 + 19?
Think
about it before you go on!

Here's the answer: again, add in parts.
Look at the example on the right.

16 + 19

=
6 +
9 + 10 + 10

=
15 + 10 + 10 = _____

4. Add in parts.

a.
13 + 18

= ____ + ____ + 10 + 10

=

b.
15 + 15

= ____ + ____ + 10 + 10

=

c.
17 + 18

= ____ + ____ + 10 + 10

=

d.
19 + 15

= ____ + ____ + 10 + 10

=

e. 18 + 12
= ______

f. 13 + 16
= ______

g. 16 + 17 = ______

h. 17 + 15 = ______

5. a. Laura owns 13 cats. Five of her
cats live in the house.
How many of her cats live outside?

b. Laura's cats eat 20 lb of cat food in a week. Laura has two 4-lb
bags at home. How many more pounds of
cat food does she
need to have enough for one week?

6. Count by threes.

42, 45, _______, _______, _______, _______,_______, _______, _______

7. Find the pattern
and continue it. This pattern “grows” at each step.

+

+

+

+

+

+

+

+

1

3

7

13

21

31

_____

_____

_____

Add two-digit numbers: Add
the tens and ones separately

Add tens on their own.
Add ones on their own.
Lastly, add the two
sums.

How to set up algebraic equations to match word problems
Students often have problems setting up an equation for a word problem in algebra. To do that, they need to see the RELATIONSHIP between the different quantities in the problem. This article explains some of those relationships.